Americans create billions of pounds of plastic waste per year. A substantial portion of this plastic waste is discarded packaging materials and water-absorbent materials such as disposable diapers and hygienic products. Unfortunately, this plastic waste is substantially non-biodegradable; in fact, it takes approximately 450 years to degrade polyethylene, a major constituent of plastic waste. Accordingly environmentally unsound and potentially hazardous methods of disposing this waste must be utilized, as for example, by landfill or incineration. Furthermore, non-biodegradable plastic is often used in garbage bags which, in addition to creating a waste problem in of themselves, are impermeable to most bacterial agents thereby preventing microbial degradation of the contents within and, thus compounding the problem of waste disposal. These problems have lead to legislation in at least 12 states requiring certain products to be made of "degradable" plastic. Accordingly, health and environmental as well as legal concerns have recently increased interest in the development of biodegradable plastics.
Prior to the present invention, the major approach to imparting biodegradability to polymers such as polyethylene has been to make physical mixtures of the polymer with modified corn starch. The corn starch degrades leaving behind a porous polyethylene matrix which, in theory, due to the increased exposed surface area, is more prone to both chemical and microbial attack. However, the porous polyethylene remains fairly stable and is not easily biodegraded. Furthermore, chemical oxidants, added to the polyethylene and cornstarch formulation to increase the rate of degradation of the porous polyethylene matrix, could in and of themselves be potentially environmentally hazardous.
Sugar-containing polymers are not entirely new. For example, sucrose has been grafted onto poly(vinyl alcohol) via chemical etherification to produce "polysugars." These polymers have been proposed as non-caloric sweeteners. Similarly, sucrose-containing polymers, as for example, polyacrylic-sucrose graft polymers have been prepared for use as biodegradable body implants. These sucrose graft polymers can be manufactured by the chemical esterification of sucrose with large polymers. Unfortunately, these sucrose derivatives have a number of undesirable properties. For example, in these sucrose-containing polymers the sucrose is either grafted onto the polymer backbone or the polymer attaches to the sucrose molecule via an ester linkage, as opposed to the actual incorporation of the sucrose into the polymer backbone. Thus, the biodegradability of the sucrose does not necessarily guarantee biodegradability of the polymer backbone.
Proctor and Gamble. Inc. has produced the so-called "sucrose polyesters" for use as fat substitutes. However, these esters are not in fact polymeric materials. Rather, these "sucrose polyesters" are sucrose molecules with high degrees of substitution with fatty acid esters. Although displaying some degree of biodegradability, it has been determined that substitution of more than five oleic acid molecules per sucrose decreases the biodegradability of the sucrose polyester. To limit the extent of substitution on the sucrose molecule, expensive and time consuming chemical blocking techniques must be used. Additionally, unless all eight hydroxyl groups of sucrose are esterified, various isomers of sucrose pehta and hexa esters are produced. Each isomer has different properties that leads to different extents of biodegradability.
Prior to the discovery of the present invention, it was difficult to manufacture di-substituted sugars necessary for the synthesis of sugar-based polymers due to the poor selectivity of chemical synthesis. The randomly substituted sugar molecules which are the product of chemical synthesis are highly undesirable for use in the synthesis of sugar-based polymers, as the resulting polymers have a large, brittle network. Such polymers are undesirable for use in most commercial products.
Methods are available to increase the regioselectivity of chemical synthesis. These methods. however, invariably involve expensive and tedious blocking and de-blocking steps. For example, sucrose contains three primary and five secondary hydroxyl groups. It is possible to chemically recognize primary groups solely via etherification with a bulky tertiary alkyl chloride such as trityl chloride. The size of the trityl group prevents any reaction at the secondary positions. This route has been used for the synthesis of sucrose-based sweeteners, yet has not been shown to be economically viable for large scale polymer synthesis.
Another limitation associated with chemical synthetic routes is the lack of control over the degree of substitution. In a typical chemical synthesis, mixtures of mono-, di-, tri-, and oligo-substituted sugar derivatives are formed. This variation and heavy degree of substitution severely impedes the synthesis of sugarbased polymers.
Clearly, if a method of acylating sugar molecules at consistent sites, as well as a method of controlling the degree of acylation were available then sugar molecules could be regioselectively acylated with an acid derivative having at least two carboxyl groups. The free carboxyl group of the resulting sugar ester could then react with a free hydroxyl group on another sugar ester to form a polymer having repeating sugar units in the polymer backbone. By limiting the degree of substitution on the sugar molecules, a polymer could be made having sugar molecules esterically bound with fatty acid linkages that could be utilized in various commercial products. Furthermore, by limiting the degree of substitution on the sugar molecules, it is contemplated that the undisturbed hydroxyl groups on the sugar molecules will cause the resulting sugar-based polymer to be water absorbent. SUMMARY OF THE INVENTION
It has been discovered that biological acylation of sugar molecules overcomes the previously discussed shortcomings associated with chemical acylation of sugar. It has been discovered that enzymes are capable of di-substituting organic acid derivatives at very specific locations on sugar molecules by diacylating the sugar molecules with organic acid derivatives having at least two carboxyl functionalities, the resulting sugar esters can be polymerized to provide a sugar-based polymer. Therefore, in accordance with the present invention, a method of manufacturing a sugarbased polymer is provided.
The present invention further provides a novel sugar-based polymer wherein the sugar is incorporated into the polymer backbone itself. Without being restricted thereto, it is theorized that the sugar-based polymer will be biodegradable in that decomposition of the sugar molecules of the polymer backbone will result in decomposition of the polymer itself.
Many commercial products are comprised of polymers. Therefore, it is contemplated that such products can be manufactured using the sugar-based polymers of the present invention. Specifically, the present invention discloses sugar-based polymers which. in theory, are biodegradable. By employing the sugarbased polymers of the present invention in the manufacture of various commercial products it is contemplated that biodegradability can be imparted to such products.
In accordance with the present invention, a novel polymer is provided which incorporates an abundantly available and recyclable resource, sugar, Thus, for example, plastic products made with the sugarbased polymers of the present invention will be based, in large part, on a renewable resource. Whereas, polyethylene, the major component of most traditional plastics, is based on the more expensive and essentially non-renewable resource, petroleum.
In general, the present invention is directed to a method of making sugar-based polymers. In particular. a sugar and an organic acid derivative having at least two carboxyl functionalities are provided. The amount of sugar and organic acid derivative provided will be such that the molar ratio of reacting carboxyl groups on the organic acid derivative to reacting hydroxyl groups on the sugar is about 1:1. An amount of a hydrolytic enzyme is further provided. The hydrolytic enzyme should be capable of regioselectively di-acylating the sugar molecules with the organic acid derivative. Finally, a substantially non-aqueous organic solvent is provided. The organic solvent must be capable of solubilizing the sugar. The organic solvent, however, must not adversely affect the catalytic activity of the hydrolytic enzyme. Additionally, the organic solvent should not hydrolyze the acylated sugars. The sugar, organic acid derivative and hydrolytic enzyme are mixed in the organic solvent. The resulting mixture is then agitated for a period of time sufficient to allow for the polymerization of the sugar.
In another aspect of the present invention, a method of making an enzyme-acid derivative intermediate useful in the synthesis of sugar-based polymers is provided. According to this method, a substantially non-aqueous organic solvent is provided, in which, the hydrolytic enzyme is catalytically active. In the substantially non-aqueous organic solvent, a hydrolytic enzyme is mixed with an organic acid derivative having at least two carboxyl functionalities.
In yet another aspect of the present invention. an enzyme-acid derivative intermediate useful in the synthesis of sugar-based polymers is provided. The enzyme-acid derivative intermediate is of the general formula: EQU E--A
wherein A comprises an organic acid derivative having at least two carboxyl functionalities, and E comprises a hydrolytic enzyme.
In another aspect of the present invention, an enzyme-organic acid derivative-sugar intermediate useful in the synthesis of sugar-based polymers is provided. The enzyme-organic acid derivative-sugar intermediate is of the following general formula: EQU E--A--S
Wherein S is a sugar selected from the group consisting of mono-, di-, tri- and oligosaccharides: A comprises an organic acid derivative having at least two carboxyl functionalities; and E comprises a hydrolytic enzyme. In another aspect of the present invention a sugar-based polymer is provided. The sugar-based polymer is of the general formula: EQU (S--A).sub.n
wherein S is selected from the group consisting of a mono-, di-, tri- and oligosaccharides: A comprises an organic acid derivative having at least two carboxyl functionalities; and n is greater than or equal to 2.